This application claims priority to Korean Patent Application No. 2004-9657 filed on Feb. 13, 2004, in the Korean Intellectual Property Office, the content of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates generally to solid-state imaging devices such as charged coupled devices for example, and more particularly to a solid-state imaging device with reduced vertical smear.
2. Description of the Related Art
A conventional solid-state imaging device, such as a charge coupled device (CCD), includes a plurality of light collectors. Each light collector (hereinafter referred to as a light sensor) forms one pixel for transforming received light into an electrical signal. The electrical signal is thereafter converted into a digital signal for further processing.
Solid-state imaging devices are classified into a horizontal overflow type and a vertical overflow type. In the horizontal overflow type, a saturated static current is discharged in a horizontal direction. In the vertical overflow type, a saturated static current is discharged in a vertical direction.
Further in the horizontal overflow type, photo-electrically converted static electrons are discharged into a transfer register in the light sensor except a depletion area therein, thereby generating a pseudo signal called as a smear phenomenon. In contrast, in the vertical overflow type solid-state imaging device, the photo-electrically converted static electrons are discharged into a substrate of the light sensor except a depletion area therein. Therefore, the smear phenomenon is relatively less generated for the horizontal overflow type than the vertical overflow type.
FIG. 1 shows a cross sectional view of a conventional vertical overflow type solid-state imaging device. Referring to FIG. 1, an N type light sensor 14, an N type vertical transfer register 16, and a P type channel stopper region 18 are formed within in a first P type well 12. The first P type well 12 is formed on an N type semiconductor substrate 10 which is comprised of silicon for example.
A P type electrical charge storage region 20 is formed on a surface of the light sensor 14, and a second P type well 22 is formed under the vertical transfer register 16. A respective transfer electrode 26 comprised of polysilicon for example is selectively formed over the vertical transfer register 16. A gate insulation layer 24 is interposed between the transfer electrode 26 and the vertical transfer register 16.
A light block layer 30 comprised of a single layer of tungsten (W) or aluminum (Al) for example is formed on the transfer electrode 26. An insulating interlayer 28 is formed between the light block layer 30 and the transfer electrode 26. A surface protection layer 32 comprised of silicon nitride for example is then deposited by a plasma-enhanced chemical vapor deposition (PE-CVD) process for example including over the light block layer 32. A P type area 34 between the light sensor 14 and the transfer register 16 serves as a read-out gate 34.
A portion of the light block layer 30 over the light sensor 14 is selectively removed by an etching process to form an opening 36 through the light block layer 30. Thus, light L is received by the light sensor 14 through the opening 36 for being sensed by the light sensor 14.
Further referring to FIG. 1, the gate insulation layer 24 and the insulating interlayer 28 that are transparent extend from a lower portion of the opening 36 to a top portion of the vertical transfer register 16. Therefore, a portion of the light L is diffracted toward the vertical transfer register 16 via the insulating interlayer 28 and the gate insulation layer 24. Such diffracted light is irradiated onto the vertical transfer register 16 to thereby generate the smear phenomenon.
Reducing a thickness of the insulating interlayer 28 between the light block layer 30 and the light sensor 14 has been suggested for preventing the generation of the smear phenomenon. However, the smear phenomenon is not sufficiently reduced.
In the prior art, the light block layer 30 is a single layer of material that is patterned by an anisotropic (i.e., directional) etching process for precisely aligning the opening 36 over the light sensor 14. Thus, the sidewalls of the light block layer 30 facing the opening 36 have a flat vertical profile. Accordingly, a portion of the light L is refracted from the edge portion of the light block layer 30, and passes to the vertical transfer register 16. As a result, in the conventional solid-state imaging device, an oblique light L1 and/or a reflection light L2 reflected between a surface of the substrate 10 and a lower surface of the light block layer 30 passes into the vertical transfer register 16 for frequent generation of smear phenomenon.